What is the purpose of making 3 priority level by WCAG?

is it like?

• If client not paying extra or if we don't have much time then go for A
• If client paying then or if we have time to make site compatible go for at least AA
• If client paying and needed according to govt. rules then go for AAA

If we are making site then which level we should we try to achieve, or we should do only on client request?

Although i found these definitions on this site but these are confusing for me

• Priority 1: For all users to access the Web content and for Web developers to attain Conformance level “A”, these requirements must be satisfied.

• Priority 2: These requirements should be satisfied by the Web developers so that no group finds it difficult to access the Web content and so as to attain Conformance level “AA”.

• Priority 3: These requirements may be satisfied by the Web developers to facilitate access to Web content for some groups and attain Conformance level “AAA”.